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Vanvelsor

'89 Reatta Speedometer Issues

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Hello everyone - I'm hoping someone out there can help me. I recently purchased an '89 Reatta, and have found that the speedometer seems to have a mind of its own! At any given point, even at a standstill, the speedometer will wander all over the place, many times just pegging at 124. I've read about a speed sensor? I thought perhaps that this could be the root of the problem, but then learned that this works in conjunction with the ABS and also the transmission. The car seems to drive fine other then the misleading speedometer readings. Also, the CC seems to be malfunctioning and I'm wondering if the two issues are related? Any help would be greatly appreciated! -Bill 

 

PS: See photo- if I were actually driving this car at 124mph, I think I'd have bigger problems! 😂

 

IMG_4760.JPG

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Welcome to the forum!  That is a strange problem that I've not heard of before.  When it happens, does the problem go away after you turn the ignition off and then back on?  The next time it happens you should go into diagnostics and see if the CRT shows the same reading as the speedometer. The speed can be displayed on the CRT by going into the Reatta's onboard diagnostics and looking at BCM data BD60.

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Welcome to the AACA Discussion Forums. I have moved your question to the Buick Reatta Forum. I am sure you will find folks here who are well equipped to answer this and any other Reatta related questions. After moving it, I see you have posted duplicates of the question I am going to remove the duplicates to prevent you having more difficulty following multiple replies to multiple discussions. 

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Thank you Ronnie, I've followed your instructions and gone into the BCM codes, found that the CRT DOES MATCH the speedometer! So strange, however, as I'm sitting with the car OFF, key switch on- running through the diagnostics, the speedometer starts creeping up! I did the process twice, as the first time- by the time I got to code BD60 car speed, the speedometer had steadied at 0. Ok, so I ran through the whole procedure again, and waited a few moments, sure enough the speedo started to run away- AS DID THE CRT! I'm not sure where this leaves me, perhaps the speed sensor still? Please advise, and once again- THANK YOU for your incredible knowledge! - Bill 

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To isolate the problem I would unplug the speed sensor and see if the problem with the speed creeping up goes away. If it does the speed sensor is probably bad. If not more digging will be needed.

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Ok, first I'll need to locate it- I believe it's connected to the transmission housing? Is that the point at which you'd suggest I unplug it? Please excuse my nativity, as I'm somewhat new at this! Also, just curious, and again- this may sound like a silly question, but if it's unplugged, wouldn't it NOT read anything?? 

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Unplug it at the speed sensor on the tranny. I would assume the cluster would read zero.

This is a weird event; let us know what you find

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Thank you for keeping at this with me- and sorry to sound like a novice, which I clearly am, but I see several sensors, I'm not exactly sure which one it is. At first I thought in would be located, or accessed from under the car, but now I'm looking more towards one of these underneath the ERG valve, and the throttle. Neither look so easy to get to- am I in the completely WRONG spot? Again, my apologies for the naïveté...

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Passenger side. Arrow marks the spot. :)

 

engine_sensor_locations.jpg

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The problem you are having is really odd so I've been reading the service manual to see how the speedometer gets it's signal. 

 

When the transmission is turning the axle it is also turning the speed sensor.  The speed sensor generates an AC voltage that is sent to the ECM. Here is where it gets complicated. -  The ECM sends a signal to the BCM to control the cruise control, AC and other things. The BCM also sends a signal to the Instrument panel AND it displays the speed on the CRT when you select BD60. So, if the speed sensor generates AC voltage when turning it couldn't be generating a voltage when it's not turning. 

 

This is a long shot  based on just a guess...  since your speedometer is going up randomly when the car is sitting still, I'm wondering if the rectifier in you alternator might be going bad and instead of sending pure DC through the electrical system continuously, it might be sporadically putting out AC voltage that the ECM or BCM is interpreting as a signal from the speed sensor. OR, you have a bad ground connection for the computers that is causing the random AC voltage.

 

Try disconnecting the speed sensor and see what happens.  If the problem is still there I would check the isolated ground connections to make sure they are clean and tight and have the alternator tested to make sure it is working properly.  Ground Terminals & Junction Box Location

 

 

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Electronics clichés happen on other cars also....... The picture below is a 2005 Rendezvous taken in 2010 in West Texas....note the RPM is at 2000 which was around 75 mph,  but the speedometer is way past 120 mph.

When we stopped overnight,  the problem never appeared again as long as I owned the car.

RAMSEY.jpg

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The possibility of a bad diode in the alternator being the issue would require the engine to be RUNNING. Was the engine running when the weird display showed?

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10 minutes ago, harry yarnell said:

The possibility of a bad diode in the alternator being the issue would require the engine to be RUNNING.

You are correct. I assumed it was running since his photo of the speedometer reading 124 didn't have any idiot lights on but that might not be the case.

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Ok, so I located the correct sensor, thanks to your diagram, unplugged it, and much to my dismay- the speedometer still wanders! Curious to the next step, but I'm assuming it's safe to say it's not the sensor then?? 🤔

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I have to think a minute, in regards to the car being running. It actually also happened when I was going through the diagnostic test too, so- in that case the key was on, but engine not running. Also, after the sensor was unplugged, I did start the car to check the status, and before I got the car out of the garage- it begin to wander. It bounces all over too, not just off by a few MPH! It often pegs at 124, which is concerning to me more because it racks up the mileage on the odometer REAL FAST that way! 😩 I will begin to trace down the grounding, but boy there sure are a lot of wires!! Once again, thanks to all who are chiming in with thoughts or ideas, especially Ronnie! 

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It's a recently purchased Reatta. Maybe it has the wrong chip in the BCM.

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Wait WHAT! the odometer is actually increasing the miles??? So the computer actually thinks the car is going 124mph ... Hmmmm

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Yes, I found it strange too, when I realized that the mileage was going up, sometimes with the car not even moving! 😩 And at 124 mph- it goes up quickly! Honestly, that's what bothers me more- I could live with the speedometer inaccuracies, however I like everything in working order. Unfortunately, I use this car as my daily driver, so I'm gonna have to hunt this issue down when time allows....

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To further isolate the problem, pull interior fuse #1 to cut power to the ECM and see if the speedometer still goes up. If it stops going up we will have to try to figure out why.

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Posted (edited)

The VSS signal is very small. I would suspect bad wiring. It has been a while but think the signal goes from the VSS to the ECM and from the ECM to the BCM which generates the speedo. Any bad connector could cause.

 

The schematic is on page 8A-34-0 et subs of the '89 FSM.

cruise.jpg

Edited by padgett (see edit history)

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Thank you for the continued support on this one! I've found one very obvious ground that looks sketchy, it's one of the larger wires coming directly off the battery, so I'm gonna inspect closer, disassemble, clean or possibly replace. This is far from the harness that feeds the VSS, so doubtful this is the issue, but looks like if not an issue, could be in he not so near future. From what I gather, the ECM and BCM are located behind the glove box, is that correct? And your stating that any wiring issue, weak ground, would be in this area- the wiring from the VSS, to the ECM, and the BCM? Also, could it even simply be a loose connection on one of these two units? Unfortunately time has not allowed me to investigate much further in the past couple days- but thank you again for hanging in there with me! I will report my findings ASAP! 

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Posted (edited)

Yes the BCM is behind the glove box and is removed that way. The ECM is also behind the glove box but is removed from under the glove box, you will have to remove a few of the vacuum lines in order to pull it out all the way - It is kinda tricky.

I hope it is the same for the 89' if not please correct me someone - Sorry if I am wrong.

 

Do you have a FSM Factory Service Manual, if not you might want to get one.

Edited by SCOTT's 90's (see edit history)

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Just wanted to check in with everyone who's helped me out so far on this problem- I, unfortunately, have not had time to continue to track this issue down, but hope to get to the ECM and BCM this weekend, to check connections and get a visual on the grounds in that area. In the meantime, I've been driving the car, speedometer still acting erratic, but what I've recently noticed is that it has been tending to read extremes. What I mean is that it often reads zero one moment, even when driving on the highways, and then it will immediately peg at 124, even at stand still. I'm not sure if this is useful info in trying to diagnose the problem, it's just hat when I first noticed this problem, it was much more gradual. Also, another thing that I've noticed, is that while the speedometer is pegged, reading 124 mph, AT A STOP SIGN, I decided to see what happen it I put the car in park, it seemed that shifting in and out of park- DID have an effect. Meaning it read 124 mph, I put the car in park, the speedometer went to zero, and the moment I shifted back to drive, the speedometer went right back to 124! I did this a couple times in a row, and witnessed the same results, before a car pulled up behind me, and I had to move along. Again, not sure if this was completely conincedental, or this may be a clue?? 

Hopeing to carve out some time this coming weekend- perhaps even solve this mystery- that would be nice! I DO have the FACTORY SERVICE MANUAL, btw, yes- it's a def nescisity! 

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